The ability to control neuronal function in cells, tissues, and living animals is critical for understanding the role of specific proteins in neural function and behavior. Controlling protein function by light has emerged as a popular approach due to its limited effects on endogenous cellular function. However, other than microbial opsins, few proteins when expressed in mammalian cells can be regulated by light. This application describes the design, construction, optimization, and application of a new class of proteins, the optoTFs, which are genetically encoded, light-activated transcription factors. OptoTFs will allow gene transcription in cells, tissue, or animals to be controlled by light in a spatially and temporally controlled manner. Experiments in this application seek to optimize the construction of an OptoTF that has CREB activity following light stimulation. The ability of light to activate CREB activity in dissociated neuronal culture and in organotypic slice culture will be optimized. Using this optoTF, the role of CREB in dendrite morphogenesis will also be investigated. The experiments in this application will develop a fundamentally novel approach to use light to spatially and temporally manipulate gene transcription. Furthermore, the methods established here are likely to be generalizable to other types of proteins, thereby potentially enabling the optogenetic control of a wide range of proteins for numerous applications. This application describes the design, construction, optimization, and application of a new class of proteins, the optoTFs, which are genetically encoded, light-activated transcription factors. OptoTFs will serve as a novel tool for the control of gene expression in cells, tissues and living animals. The development of this tool will enable future studies to establish the role of gene transcription in learning, memory, and addiction.